1. Field of the Invention
The present invention relates to optical wavelength control systems and was devised by paying specific attention to the possible use in optical communication systems. However, reference to this preferred field of use must in no way be construed as limiting the scope of the invention.
2. Brief Description of Related Developments
Commercial WDM (Wavelength Division Multiplex) transmission systems, such as “dense” WDM (DWDM) systems provide high transmission capacity by using reduced channel spacing (e.g. 100–50 GHz). Real time monitoring and control is thus necessary in order to ensure the channel peak wavelength stability required for the optical sources used in such systems.
A number of devices adapted for that purpose (and primarily for wavelength monitoring) are based on the arrangement currently referred to as “wavelength locker”. This usually consists of two photodiodes sampling two portions of the optical beam (typically a laser beam). One of the photodiodes, used as a reference, samples an unfiltered portion of the laser beam. Another portion of the laser beam is passed through an optical filter and caused to impinge onto the second photodiode. The response (i.e. the photocurrent) of the first diode is thus indicative of the power emitted by the optical source; the response of the second diode is a function of the possible displacement of the actual wavelength of the beam generated by the laser source with respect to the wavelength of the filter.
A beam splitter is used to split the laser beam into a main beam to be used for the intended application (e.g. for launching into a fiber) and one or more secondary beam or beams to be directed towards the photodiodes of the locker arrangement.
Various arrangements are known in order to effect stabilisation. For instance, in the case of diode lasers, a Peltier element can be used as a wavelength stabilising element by controlling the temperature of the laser diode, while power stabilisation is effected by controlling the laser bias current.
Arrangements of the general type referred to in the foregoing, or substantially similar thereto, are disclosed e.g. in U.S. Pat. No. 5,825,792, U.S. Pat. No. 6,094,446 and U.S. Pat. No. 6,377,592 B1.
Specifically, the arrangement of U.S. Pat. No. 5,825,792 comprises a narrow bandpass, wavelength selective transmission filter element, of Fabry-Perot etalon structure, through which a non-collimated beam from a laser source is directed onto two closely spaced photodetectors. For wavelength stabilisation, the differential output of the two photodetectors is used in a feedback loop to stabilise the wavelength of the laser source to a desired target wavelength. Through the angular dependence of wavelength transmission of the Fabry-Perot etalon, the wavelength variation from the source is converted to a transmission loss, which is different for the two photodetectors, so that the wavelength change is detected as a differential power change. The device functions as an optical wavelength discriminator in which the detectors convert optical energy to current for a feedback loop for controlling the light source. A lens may be used to control the divergence of the light incident on the filter element to optimise power transfer. Optionally, wavelength tunability is provided by changing the angle of inclination of the Fabry-Perot etalon relative to the laser source.
In the arrangement of U.S. Pat. No. 6,094,146 the light emitted by a laser diode is propagated towards an interference optical filter. Light passing through the filter and the light reflected therefrom are caused to impinge onto two photodiodes to generate respective output signals. The ratio of those signals is calculated in an arrangement including an adder, a subtractor and a divider. The arrangement further includes an error detector adapted to detect the difference between the output ratio and a reference value. The emission wavelength of the laser diode is controlled in such a way that the error signal may be equal to zero.
In the arrangement of U.S. Pat. No. 6,377,592 B1 the light emitted by a laser diode is propagated towards wavelength-neutral power dividers implemented in the form of two semitransparent mirrors as surfaces of the same body of transparent material such as glass.
Somewhat similar arrangements are also known from U.S. Pat. No. 5,781,572, U.S. Pat. No. 6,384 947, EP-A-1 218 983 and JP07095159.
A number of factors must be taken into account in applying such arrangements in order to produce compact stabilised optical sources.
Generating optical signals proportional to the optical power and wavelength of a laser source almost invariably requires the radiation from the laser source to be split over distinct propagation paths. This may turn out to be a fairly critical solution, especially when the laser beam emitted from the back facet of the laser is exploited for stabilisation purposes as an alternative to splitting a fraction of the main beam generated from the front facet of the laser.
In order to collect sufficient power, the light signal must be collimated into a low-divergence beam by using a lens. This arrangement necessitates a critical active alignment step, as recognised e.g. in K. Anderson, IEEE Electronic Component and Technology Conference, 1999, pp. 197–200.
Additionally, the wavelength selective components must be temperature controlled in order to avoid drifts in the wavelength locking point generated by temperature changes.
Also, the stabilization system must be compact and adapted to be included in the same package of the laser source thus tackling the related problems in terms of optical coupling, space requirements (small “footprint”) and power dissipation.